The present invention generally relates to laser-based systems useful in construction applications and, more particularly, to a laser-based system for measuring the position of a receiver, mounted on mobile construction equipment operating at a work site, with respect to a transmitter located at a stationary reference position at the work site. The position of the receiver is determined in three dimensions by the system.
In prior laser-based systems, such as that disclosed in U.S. Pat. No. 3,588,249, for example, a reference plane is established throughout a work site by a transmitter which emits laser energy in a level reference plane. The reference plane is typically established by a laser beam which is projected radially outward from the transmitter and rotated continuously through 360 degrees to sweep around the entire work site. One or more receivers may be employed throughout the work site to sense the location of this reference plane. Such receivers may be mounted on a surveyor rod as described in U.S. Pat. No. 4,030,832, or they may be employed as part of a control system for construction or agricultural equipment as described in U.S. Pat. Nos. 3,813,171; 3,873,226; 3,997,071; and 4,034,490.
Prior laser systems have provided an indication of elevation throughout the work site, but they have not typically indicated the position of the receiver within the work site. Thus, for example, the height of the blade on a road grader with respect to the reference plane can be measured by such prior systems, but if this height is to vary over the work site it is necessary for the operator to determine by some other means where the road grader is located within the site in order to know precisely what the elevation should be at that location. In other words, prior laser systems provide only one dimension of position information, that being elevation.
The laser survey system disclosed in the above cross-referenced patent application provides not only elevation information, but also position information in two other axes. The system includes a laser transmitter, located at a reference position at the work site, which sweeps a laser beam radially in a reference plane. The system includes a receiver, located on mobile earthmoving equipment operating at the work site, which has a sensor that determines the relative elevation of the laser reference plane. The receiver also includes a pair of reflectors, each of which reflects laser energy back to the transmitter. The reflectors are oriented by an appropriate control system such that a reference line extending between them is generally perpendicular to the beam as the beam sweeps across the reflectors. The laser transmitter has a sensor which receives the reflected laser energy, and, in response thereto, produces receiver position information for transmission to the receiver.
The laser transmitter is designed to rotate the laser beam continuously through 360 degrees at a substantially constant annular velocity and thus sweep the beam past the two reflectors of the receiver once during each revolution. During each revolution of the laser beam, the transmitter receives back two short bursts or pulses of laser energy from the two reflectors. Thus, since the laser beam sweeps at a substantially constant angular velocity and the distance between the reflectors is fixed, the time period between receipt of these two pulses provides a highly accurate basis for the calculation of the range, or distance of the receiver from the transmitter. The orientation of the beam at the time that the two pulses are received defines the direction of the receiver with respect to the transmitter. This may be detected by an angle encoder or similar arrangement on the transmitter.
It is realized that the accuracy of the range calculation is dependent upon a uniform rotational velocity for the laser beam. Should conditions unexpectedly arise under which operation of the transmitter introduces an unacceptable degree of variability in the laser beam angular sweep velocity, the accuracy of the range calculation will decrease accordingly.
Consequently, a need exists for an alternative approach to determining the range or distance of the receiver from the transmitter. An approach is desired in which the accuracy of the range calculation does not depend upon maintaining a substantially constant angular velocity of the laser beam, and thus reducing the need for highly precise mechanical operation of the transmitter.